1. Field of the Invention
This invention relates to a frost and dew sensor for use in a defroster of a refrigerator, air-conditioner and various other industrial appliances.
2. Description of the Related Art
It has been a common knowledge that under certain conditions, the surface of a heat-exchanger, incorporated in a refrigerator, an air-conditioner or a similar apparatus, is covered with frost and ice. Continued operation of the heat-exchanger covered with frost and ice would remarkably reduce the energy efficiency, which is uneconomical and occasionally causes a failure or fault.
Heretofore, attempts were made to detect the building-up of frost and dew; some of the proposed detecting means used a resonator, some utilized the change of dielectric constant of an element due to the developing of frost or dew, and others were optical type.
FIGS. 13 through 16 of the accompanying drawings illustrate two prior art sensors each using a resonator; one sensor detects the change of resonance frequency of a resonator, and the other detects the change of amplitude of a resonator.
In FIG. 13, a piezo-electric resonator 14 is supported on the upper surface of a tubular housing 10 via a resilient support 12 and bears a pair of electrodes 16a, 16b attached one on each side of the resonator 14, and a pair of output terminals 18a, 18b leading from the electrodes 16a, 16b, respectively.
FIG. 14 is a circuit diagram of the sensor of FIG. 13, in which the output of the resonator 14 is supplied to a resonance-frequency discriminator 22 via a resistor R on one side and an amplifier 20 on the other side where the output is amplified as a matter of fact. Then the output of the discriminator 22 is taken out to the exterior. In operation, as frost or dew develops over the surface of the resonator 14, the resonance frequency derived from the resonator 14 varies depending on the amount of frost or dew built up. When the extent of change in resonance frequency climbs over a predetermined value, this sensor discriminates or judges that the resonator 14 has been covered with frost or dew.
The sensor of FIG. 15 is of the type in which the developing of frost or dew is detected based on the change of amplitude of a resonator 114. This sensor is identical in basic construction with that of FIG. 13; but, the output of the resonator 114 is supplied to an oscillation amplitude discriminator 124, as shown in FIG. 16. In this sensor, as frost or dew develops over the surface of the resonator 114, the oscillation of the resonator 114 is restricted depending on the weight of frost or dew grown up. Thus when the extent of change in amplitude ascends beyond a predetermined value, this sensor presumes that the surface of the resonator 114 has been covered with frost or dew.
FIGS. 17 to 19 are diagrams showing various wave forms of the oscillation outputs of the piezo-electric resonator 114 and of the detected outputs of the sensors shown in FIGS. 13 through 16.
Specifically, FIG. 17 shows a wave form of the oscillation output of the sensor of FIGS. 13 and 14, indicating that at a time point t.sub.1, concurrently with the developing of frost, the resonance frequency increased about two times. FIG. 18 shows another wave form of the oscillation output of the sensor of FIGS. 15 and 16; it can be observed that concurrently with the developing of frost or dew (time point t.sub.1), the amplitude of the output signal derived from the resonator 114 is reduced.
When any change of the oscillation frequency or amplitude has thus been found, the sensor outputs a signal giving a notice that the resonator 14, 114 has been covered with frost or dew, in response to which generally a defroster or a dehumidifier is energized.
FIGS. 20 to 22 illustrate another prior art sensor of the type utilizing the change of dielectric constant to detect frost and dew. FIGS. 20 and 21 show the inside structure and outside appearance, respectively, of the sensor; a resistor film 230 is coated over the surface of an insulating substrate 228 on which a pair of comb-shaped electrodes 226, 226 is printed. FIG. 22 shows a detector circuit of the sensor; an alternating voltage from an alternating signal source 134 is impressed to a detection unit 232 having the construction of FIGS. 20 and 21, and the output of the detection unit 232 is supplied to an impedance detector circuit 236, the output terminal of which is connected to a non-illustrated defroster or dehumidifier.
With this prior arrangement, as frost or dew develops over the surface of the detection unit 232, the alternating impedance between the two comb-shaped electrodes 226, 226 varies. When the impedance detector circuit 236 detects this change in the impedance, it presumes that the surface of the detection unit 232 has been covered with frost or dew.
FIGS. 23 and 24 illustrate an optical type of prior art sensor.
Specifically, FIG. 23 shows the principle of operation of the sensor having a light-emitting element 338 and a light-receiving element 340; light from the light-emitting element 338 reflects on a reflection surface 342 and then strikes on the light-receiving element 340. As frost or dew develops over the reflection surface 342, the refractive index of the light from the light-emitting element 338 or the angle of incidence of the light falling on the light-receiving element 340 deviates so that the amount of light falling on the light-receiving element 340 is reduced. When any change of the light amount is thus found, the sensor makes a judgment that the surface of the reflection surface 342 has been covered with frost or dew.
FIG. 24 show the principle of operation of the sensor having an LED (light-emitting diode) 438 and a photodiode 440 receptive of the light from the LED 438. As frost or dew develops on a path of light spanning between the LED 438 and the photodiode 440, the amount of light to reach the photodiode varies. When the extent of change in the light amount is compared with a reference value in a level discriminator 444 and is thus found over the reference value, the level discriminator 444 issues a notice that the path of light between LED 438 and the photodiode 440 has been at least partly blocked by frost or dew grown up.
With the foregoing prior arrangement, the following problems are unavoidable so that adequate usefulness cannot be achieved.
Each of the known sensors of FIGS. 13 through 16, in which a piezo-electric resonator is used, tends to operate incorrectly due to the dust or other foreign matter stuck to the resonator or due to vibrations exerted on the resonator interiorly and exteriorly of the sensor.
In the known sensors of FIGS. 20 through 24, some utilizing the change in dielectric constant and others adopting an optical method, partly since it is difficult to reduce the detection unit into a compact size, and partly since the circuit structure is too complex, maintenance on a periodical basis is essential to keep the detection precision at a predetermined level. Accordingly, it is difficult not only to achieve reproducibility, but also to reduce the cost of production.